This application is based upon and claims the benefit of priority from the prior Japanese Patent Applications No. 11-135821, filed May 17, 1999; No. 11-163482, filed Jun. 10, 1999; No. 11-354417, filed Dec. 14, 1999; and No. 2000-121198, filed Apr. 21, 2000, the entire contents of which are incorporated herein by reference.
The present invention relates to a near field optical microscope and a probe for the near field optical microscope which detect light scattered by a probe entering into a near field to obtain information concerning the surface of a sample.
A scanning probe microscope (SPM) is a generic name of an apparatus in which, when a probe is set 1 xcexcm or less close to a sample surface, the probe is let scan in the X- and Y-directions or X-, Y-, and Z-directions while detecting a correlative effect caused between both the probe and the sample surface, thereby to carry out two-dimensional mapping of the correlative effect. For example, scanning probe microscopes include a scanning tunneling microscope (STM), an atomic force microscope (AFM), a magnetic force microscope (MFM), and a scanning near-field optical microscope (SNOM).
Among them, developments of the SNOM as an optical microscope having a resolution which exceeds the diffraction limit by detecting near field light formed near a sample has been eagerly promoted after the later half of 1980s to achieve application use for fluorescence measurement of a bionic sample, evaluation of an element (various characteristic evaluations of dielectric light guide paths, measurement of light emission spectrums of semiconductor quantum dots, evaluation of various characteristics of semiconductor laser, etc.).
The SNOM is basically an apparatus which sets a sharp probe near a sample with light illuminated thereon and detects a field (near field) of light near the sample.
The U.S. Pat. No. 5,272,330 granted to Bezig et al. on Dec. 21, 1993 discloses a SNOM in which light is introduced to a probe having a narrowed top end, thereby to generate a field of light localized near a very small opening at the top end of the probe, and this is brought into contact with a sample, to illuminate a very small part of the sample. Transmitted light is detected by an optical detector provided below the sample, and two-dimensional mapping of an intensity of transmitted light is carried out.
The SNOM uses a rod-like probe such as an optical fiber or glass rod which has a top end processed to be narrow or a crystalline probe.
A rod-like probe covered with a metal film except the top end thereof has already been commercially available as an improved type of the probe.
An apparatus using this probe has an improved resolution in the lateral direction in comparison with an apparatus using a probe not coated with metal.
Meanwhile, the AFM has been most widely spread as an apparatus for obtaining topography information of the sample surface among SPMs.
The AFM detects a displacement of a cantilever which shifts in accordance with a force acting on a probe when the prove supported on the top end of the cantilever is set near a sample surface, for example, by an optical displacement sensor, thereby to obtain indirectly information concerning concaves and convexes of the sample surface.
One of the AFMS is disclosed in the Japanese Patent Application KOKAI Publication No. 62-130302.
The technique of measuring concaves and convexes on a sample by detecting a correlative force between the sample and the top end of the probe is utilized for other SPM apparatuses and is used as a means for carrying out so-called regulation.
N. F. van Hulst et al. has proposed a new SNOM which uses an AFM cantilever made of silicon nitride and detects optical information of a sample while measuring concaves and convexes of the sample by AFM measurement, in xe2x80x9cAppl. Phs. Lett. 62(5)xe2x80x9d, P. 461 (1993).
In this apparatus, the sample is set on an internal total reflection prism and the sample is illuminated with a He-Ne laser beam from the total reflection prism side, so the sample is excited and an evanescent optical field is formed near the sample surface.
Subsequently, a probe supported on the top end of the cantilever is inserted in the evanescent optical field, and evanescent light as a localized wave is converted into scattered light as a propagation wave. A part of this light is propagated inside a silicon-nitride-made probe which is substantially transparent with respect to the He-Ne laser beam and passes to the back side of the cantilever.
This light is condensed by a lens provided above the cantilever and enters into a photomultiplier tube through a pinhole provided at a position conjugate with the top end of the probe with respect to the lens. A SNOM signal is outputted from the photomultiplier tube.
While detecting this SNOM signal, displacement of the cantilever is measured by an optical displacement detection sensor. For example, a piezoelectric scanner is subjected to feedback control such that the displacement is maintained at a regulated constant value.
Accordingly, during one scanning, SNOM measurement is carried out based on a scanning signal and a SNOM signal and AFM measurement is carried out based on a scanning signal and a feedback control signal.
In the SNOM of an aperture type disclosed by Betzig et al., the probe should be subjected to metal coating to obtain a high resolution in the lateral direction.
However, it is not easy to manufacture uniformly a large quantity of probes each having an opening at the top end and coated with metal
A resolution exceeding a resolution which can be realized by an ordinary optical microscope is required for a SNOM which is expected to have a super resolution. To realize the super resolution, the diameter of the opening at the top end of the probe must be 0.1 xcexcm or less or preferably 0.05 xcexcm or less.
An opening having a diameter of these values is very difficult to prepare with excellent reproductivity.
In addition, since the amount of light which enters into the probe through the opening decreases in proportion to square of the radius of the opening, the light amount to be detected is reduced so that the S/N ratio is deteriorated, if the opening diameter is reduced for the purpose of improving the resolution of an SNOM image in the lateral direction. Thus, there is a problem of trade-off.
Hence, a proposal has been made for a new SNOM (scattering-type) SNOM which uses a feature that high-diffraction dielectric material or metal strongly scatters near-field light without forming an opening at the top end of the probe.
In this SNOM, no opening is required at the top end of the probe so that there is not the problem of trade-off and the difficulty of forming the opening.
Kawata et al. disclose a scattering-type SNOM in the Japanese Patent Application KOKAI Publication No. 6-137847.
In this SNOM, the evanescent light formed on the sample surface is scattered by a needle-like probe and is thereby converted into propagation light. This propagation light, i.e., scattered light is detected by a condenser lens and a photodetector provided in a side of the probe, and optical information is obtained, based on a detection signal thereof.
Further, Kawata et al. disclose an apparatus in which a metal probe of the STM is used as its probe and propagation light generated due to scattering of evanescent light generated on the sample surface by the top end of the metal prove is observed from the lateral side of the sample and probe while controlling the distance between a sample and the probe by the STM, so STM observation and SNOM observation can be achieved, in xe2x80x9cDAI-42-KAI NIHON OHYOH BUTSURIGAKU KANKEI RENGOH KOENKAI (preliminary report compilation No. 3, page 916, March 1995)xe2x80x9d.
Also, Kawada et al. further reports that the SNOM observation can be achieved even by multiple scattering of propagation light entering obliquely from the upside of the sample, between the top end of a metal probe and a sample, in place of the evanescent light, in xe2x80x9cDAI-43-KAI NIHON OHYOH BUTSURIGAKU KANKEI RENGOH KOENKAI (preliminary report compilation No. 3, page 916, March 1996)xe2x80x9d.
Bachelot et al. also report a scattering-type SNOM depending on propagation light from the upside without using the probe having an opening, in xe2x80x9cOpt. Lett. 20 (1995), p. 1924xe2x80x9d.
Toda et al. disclose a scattering-type SNOM which uses an micro cantilever made of silicon for AFM so as to use a dark field illumination system.
Since an AFM image with a high resolution can be obtained by this AFM cantilever made of silicon and also the diffraction rate is high, the scattering efficiency of light is high so advantages can be obtained for the scattering type SNOM probe.
The scattering type SNOM probe is constructed in a structure in which the probe becomes fatter from the top end toward the bottom portion supporting the probe.
In addition, light is scattered by the part within a range of one wavelength from the top end of the probe.
Therefore, light is scattered not only by the top end of the probe but also by a part which is fatter than the top end.
A signal depending on the scattered light from the fatter part lowers the high-resolution performance of an SNOM image.
Also, since the cantilever made of silicon for AFM described above has a structure in which the probe projects from the top end of a plate-like lever, there is a drawback that scattered light is blocked by the lever part so that the scattered light cannot be used efficiently.
Angular resolution of the scattered light is also a factor important to attain high resolution. The angle which can be used is limited due to the reasons described above.
In addition, in many of existing apparatuses, an objective lens used for both the optical microscopic observation and scattering signal collection is provided above a sample, and therefore, there is a drawback that an apparatus for measurement depending on the angle of scattered light cannot be provided.
In the scattering type near field optical microscope (scattering type SNOM) utilizes a feature that light illuminated on a probe causes strong scattering at the top end portion due to an effect of electric field concentration to a sharp top end of a probe, to illuminate a very small portion of a sample in close contact to the probe with the scattered light. By taking in this light while scanning the sample, an image exceeding the diffraction limit is obtained.
With respect to the shape of the probe, probes for AFM and STM have been conventionally used but attention has not been particularly paid except the condition that the top end is sharp.
A probe for the scattering type SNOM has a structure in which the probe is fatter from the top end toward a portion closer to the base part supporting the top end. Although light is scattered most strongly by the sharp portion at the top end of the probe, the light is also scattered by the part within one wavelength or so from the top end.
Therefore, light is scattered not only by the top end of the probe but also by the part which is fatter than the top end.
A signal depending on the scattered light from the fatter part deteriorates the high-resolution performance of a SNOM image.
Sugiura et al. disclose a SNOM which uses a very small gold globe supported by a laser trap, as a scattering probe, in xe2x80x9cOpt. Lett. 22 (1997), P. 1663xe2x80x9d.
In this SNOM, drawbacks as described above need not be considered but the force of holding the very small gold globe is weak so that imaging takes a very long time.
Also, samples that can be observed are limited because of underwater operation.
The present invention has an object of providing a near-field optical microscope as a SNOM which can obtain a high-resolution SNOM image and can operate in the air be extremely excluding influences from scattered light from portions other than a probe, and the probe for the near-field optical microscope.
Also, the present invention has an object of providing a near-field optical microscope which can detect scattered light over a wider angle range by arranging the structure of a cantilever, and the cantilever for the near-field optical microscope.
To achieve the above objects, according to the present invention, there is provided a near-field optical microscope comprising: an illumination part for illuminating a sample surface with light; a probe provided at a position near the sample surface illuminated with the light; a light detection part for detecting light scattered by the probe; and a scanning part for scanning the sample and a top end of the probe relatively to each other, wherein the top end of the probe is a top end of an extending part extending in one direction from a body of the probe, in a side of the top end of the extending part, the extending part is at most three times or less as thick as a tope end diameter, over a length of a wavelength of the illuminating light, and the near-field optical microscope further comprises means for vibrating the probe in a lengthwise direction of the extending part.
Also, to achieve the above objects, according to the present invention, there is provided a probe used for a near-field optical microscope, comprising: a probe body; and an extending part extending in one direction from the probe body, wherein in a side of a top end of the extending part, the extending part is at most three times or less as thick as a top end diameter, over a length of 700 nm from the top end.
Also, to achieve the above objects, according to the present invention, there is provided a near-field optical microscope comprising: an illumination part for illuminating a sample surface with light; a cantilever having a probe and a probe hold part, with a top end part of the probe positioned near the sample; and an objective optical system for receiving scattered light generated at a top end part of the probe and caused from the illuminating light, wherein the top end part of the probe is in a view field of the objective optical system without being shielded.
Also, to achieve the above objects, according to the present invention, there is provided a cantilever which can be used for a near-field optical microscope, comprising: a hold part extending in one direction; and a probe positioned at an end part of the hold part, wherein a top end of the probe is positioned further outside a top end of the hold part in the direction in which the hold part extends.
Also, to achieve the above object, according to the present invention, there is provided a near-field optical microscope comprising: light illumination means for illuminating a sample surface of a sample with light; a probe having a top end provided at a position near the sample surface which is illuminated with the light; light detection means for detecting scattered light scattered by the probe; and scanning means for scanning the sample and the top end of the probe relatively to each other, wherein a top end diameter of the prove is equal to or smaller than xc2xc of a wavelength of the light illuminating the probe, and where a length of a range illuminated with the light from the light illumination means is z0, the wavelength of the illuminating light is xcex, a maximum value of a diameter of the probe is dmax within a range from a top end of the probe to a distance z0, and the top end diameter of the probe is d0, the diameter of the probe monotonously increases from the top end to the distance z0, and dmaxxe2x89xa7d0(z0+xcex/2)/(xcex/2) is given.
Also, to achieve the above object, according to the present invention, there is provided a near-field optical microscope comprising: light illumination means for illuminating a sample surface of a sample with light; a probe having a top end provided at a position near the sample surface which is illuminated with the light; light detection means for detecting scattered light scattered by the probe; and scanning means for scanning the sample and the probe relatively to each other, wherein a top end diameter of the prove is equal to or smaller than xc2xc of a wavelength of the light illuminating the probe, and where a length of a detection range of the light detection means is z0, the wavelength of the illuminating light is xcex, a maximum value of a diameter of the probe is dmax within a range from a top end of the probe to a distance z0, and the top end diameter of the probe is d0, the diameter of the probe monotonously increases from the top end to the distance z0, and dmaxxe2x89xa6d0(z0+xcex/2)/(xcex/2) is given.
Also, to achieve the above objects, according to the present invention, there is provided a probe used for a near-field optical microscope, comprising: a probe body; and an extending part extending in one direction from the probe body, wherein a top end diameter of the extending part is equal to or smaller than xc2xc of a wavelength of light illuminating the probe, and where a length of a range illuminated with the light from the extending part is z0, the wavelength of the illuminating light is xcex, a maximum value of a diameter of the extending part is dmax within a range from a top end of the extending part to a distance z0, and the top end diameter of the extending part is d0, the diameter of the extending part monotonously increases from the top end to the distance z0, and dmaxxe2x89xa6d0(z0+xcex/2)/(xcex/2) is given.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.